Chapter 27: Evolutionary Genetics Student Learning Objectives Upon completion of this chapter you should be able to: 1. Understand what the term species means to biology. 2. Recognize the various patterns of speciation and the factors that contribute to each. 3. Understand the concept of phylogenetic trees, and the basic principles of their formation. 4. Analyze phylogenetic trees. 5. Know the principles of the neutral theory of evolution. 6. Understand the concept of the molecular clock. 7. Recognize the key role that certain proteins have played in promoting the morphological changes that occur during evolution. 27.1 Origin of Species Overview The final chapter of the text examines the phenotypic changes that occur during evolution and their underlying genetic causes. The first section focuses on the general features of evolution as it occurs in natural populations over time. It begins with a discussion of the most commonly used characteristics to distinguish species. These include morphological traits (Figure 27.1), ability to interbreed (Table 27.1), molecular features, ecological factors, and evolutionary relationships. The section then turns to speciation the process by which new species are formed via evolution. It may occur by anagenesis, when one species evolves into a new one, or more commonly by cladogenesis, in which a species diverges into two or more different species (Refer to Figure 27.2). The section ends with a discussion of the three forms of cladogenesis: allopatric, parapatric, and sympatric speciation. Outline of Key Terms Biological evolution Macroevolution Microevolution Molecular evolution Species Subspecies Ecotypes Species concepts Biological species concept Reproductive isolation Prezygotic mechanisms Postzygotic mechanisms Evolutionary lineage concept General lineage concept Speciation Anagenesis Cladogenesis Allopatric speciation Founder effect Parapatric speciation Hybrid zones Sympatric speciation 334
Focal Points Concepts of species (pages 681-684) Types of reproductive isolation (Table 27.1) Comparison between anagenesis and cladogenesis (Figure 27.2) Allopatric, parapatric, and sympatric speciation (Table 27.2) Comparison of crosses between three species with different ploidy levels (Figure 27.4) Exercises and Problems For questions 1 to 6, choose the term that best describes the statement. 1. Each species is a population of an independently evolving lineage. 2. Examples include habitat isolation and mechanical isolation. 3. Examples include hybrid inviability and hybrid sterility. 4. A single species is transformed into another species over time. 5. A group of individuals who have the potential to interbreed and produce viable, fertile offspring. 6. The division of an original species into two or more separate species. a. biological species concept b. prezygotic isolating mechanisms c. cladogenesis d. general lineage concept e. postzygotic isolating mechanisms f. anagenesis For questions 7 to 12, choose the pattern of divergent evolution that is best described by the statement. a. parapatric speciation b. sympatric speciation c. allopatric speciation 7. May occur as a result of founder effect. 8. Occurs as a result of partial separation of two species. 9. Polyploidy in plants is an example. 10. Occurs with two species that occupy the same habitat or range. 11. Occurs as a result of geographic separation. 12. Hybrid zones are a characteristic of this model. 334
27.2 Phylogenetic Trees Overview Phylogeny refers to the sequence of events involved in the evolutionary development of a species or group of species. A phylogenetic tree is basically a diagram that describes a phylogeny (Figure 26.6). This section examines the construction and analysis of phylogenetic trees. The text focuses on cladistics (pages 739-741), which compares traits (or characters), that are either shared or not shared by different species. The text also describes how the validity of possible trees can be evaluated by the principle of parsimony, maximum likelihood, and Bayesian methods. The discussion then moves to how phylogenetic trees have helped refine our understanding of evolutionary relationships between organisms. For example, analysis of 16S and 18S rrna sequences by Carl Woese in 1977, enabled him to discover a new domain of life called Archaea (Figure 26.9). Moreover, recent molecular genetic data have shed new light on the classification of species (Figure 26.10). Finally, the section addresses the role of horizontal gene transfer in the evolution of life (Figure 26.11). Outline of Key Terms Phylogeny Systematists Phylogenetic tree Monophyletic group Clade Homology Homologous genes Cladistic approach Cladogram Ancestral (primitive) characters Shared derived characters Synapomorphy Ingroup Outgroup Principle of parsimony Maximum likelihood Bayesian methods Bacteria Archaea Eukarya Supergroups Vertical evolution Horizontal gene transfer Focal Points How to read a phylogenetic tree (Figure 27.5) Cladistics (pages 689-690) and phenetics (pages 690-692) A modern cladogram for eukaryotes (Figure 27.9) A revised view regarding the evolution of life (Figure 27.10) 335
Exercises and Problems Complete the following sentences with the most appropriate word or phrase. 1. A monophyletic group, also known as a, is a group of species consisting of all descendents of the group s most recent common ancestor. 2. The three domains of life are the,, and. 3. The movement of genes between species is called. 4. In a cladogram, an is a monophyletic group in which we are interested. 5. According to the modern cladogram of eukaryotes, both fungi and animals belong to the Opisthokonta. 6. In the cladistic approach, shared derived characters are also termed. 7. The UPGMA method provides a relatively simple method to construct a tree using the phenetics approach. UPGMA stands for. 27.3 Molecular Evolution and Molecular Clocks Overview Molecular evolution refers to patterns and processes associated with evolutionary change at the molecular level. This section begins by addressing the topic of homologous genes. Familiarize yourself with the concepts of orthologs and paralogs (Figures 27.11 and 27.12). The discussion then turns to the neutral theory of evolution (page 696). This theory suggests that most variation in gene sequences is due to mutations that are neutral or nearly neutral with regard to the phenotype. These variations accumulate in populations mainly due to genetic drift. Indeed, they do so at a relatively constant rate, which can act as a molecular clock on which evolutionary time can be measured (Figure 27.13). The section ends by highlighting how evolution also involved changes in chromosome structure and number (Figures 27.14 and 27.15). Outline of Key Terms Molecular evolution Homologous genes Orthologs Paralogs Gene family Nonneutral mutation Neutral theory of evolution Molecular clock Synteny groups Focal Points The concept of orthologs and paralogs (Figures 27.11 and 27.12) The neutral theory of evolution (page 696) The concept of molecular clock (Figure 27.13) Comparison of chromosome banding patterns among humans and apes (Figure 27.14) Synteny groups in cereal grasses (Figure 27.15) 335
Exercises and Problems Complete the following sentences with the most appropriate word or phrase: 1. The constant rate of molecular evolutionary change can be measured by the. 2. Homologous genes found in different species are termed. 3. Homologous genes found in the same species are termed. 4. A mutation is one that affects the phenotype of an organism and thus can be acted on by natural selection. 5. The of evolution has also been called the survival of the luckiest. 6. mutations have a minimal effect on the phenotype they may be slightly beneficial or slightly detrimental. 7. groups are chromosomal regions of two or more different species that contain the same groups of linked genes. 8. In comparison to, human chromosome number is formed by a fusion of two smaller chromosomes, while human chromosome number lacks a large that flipped the arrangement of bands in the centromeric region. 27.4 Evo-Devo: Evolutionary Developmental Biology Overview The last section of the chapter (and the textbook) introduces a relatively new field of biology called evolutionary developmental biology or evo-devo for short. The aim of this field is to compare the development of different species to understand the genes and mechanisms that bring about evolutionary change. The discussion focuses on the key roles that genes have played in the evolution of: 1) bird foot morphology (the BMP4 and gremlin genes); 2) animal body plan (Hox gene cluster); and 3) eyes (Pax6 gene). Outline of Key Terms Evolutionary developmental biology Evo-devo Focal Points The role of cell-signaling proteins in the morphology of birds feet (Figure 27.16) Hox gene composition in different types of animals (Figure 27.17) Eye evolution (Figure 27.19) 337
Exercises and Problems Complete the following sentences with the most appropriate word or phrase: 1. In birds feet, the protein causes programmed cell death. 2. In birds s feet, the protein inhibits programmed cell death. 3. Evolution of animal body plans is related to changes in gene number and expression. 4. The explosion, which occurred from 533 to 525 million years ago, saw a phenomenal diversification in the body plan of invertebrate species. 5. In humans, a mutation in the Pax6 gene results in an eye (iris) disorder called. 6. The gene of Drosophila is homologous to the Pax6 gene of humans. 7. The field of evolutionary developmental biology is informally referred to as. Chapter Quiz 1. The most prevalent model of speciation is believed to be speciation. a. sympatric b. parapatric c. horizontal d. allopatric 2. A mutation that does not affect the phenotype of the individual is called a mutation. a. stabilizing b. selective c. neutral d. nonsense 3. Which of the following is NOT a domain of life? a. Bacteria b. Animalia c. Archaea d. Eukarya e. all of the above are domains 4. Hybrid zones are associated with which of the following? a. sympatric speciation b. parapatric speciation c. allopatric speciation d. artificial selection 5. Which of the following is a method used to construct phylogenetic trees? a. Phenetics b. Cladistics c. Both A and B d. Neither A nor B 338
6. Which of the following are methods used to discriminate among possible phylogenetic trees? a. maximum likelihood b. Bayesian methods c. principle of parsimony d. A and B e. A, B, and C 7. Which of the following statements about horizontal gene transfer is FALSE? a. explains origin of mitochondria b. explains origin of chloroplasts c. was prevalent during early stages of evolution when all organisms were unicellular d. continues to be a prominent factor in the speciation of eukaryotes e. Choose this answer if all of the above are true 8. The anatomical structures of genitalia prevent mating between species. This is an example of what isolating mechanism? a. mechanical isolation b. sexual isolation c. gametic isolation d. hybrid sterility e. hybrid breakdown 9. In the interdigit regions in ducks, the protein inhibits BMP4, thus causing webbed feet. a. Pax6 b. Hox c. Gremlin d. Goblin e. Calmodulin 10. Which of the following are example of species concepts? a. evolutionary lineage concept b. biological species concept c. ecological species concept d. A and B e. A, B, and C Answer Key for Study Guide Questions This answer key provides the answers to the exercises and chapter quiz for this chapter. Answers in parentheses ( ) represent possible alternate answers to a problem, while answers marked with an asterisk (*) indicate that the response to the question may vary. 27.1 1. d 2. b 3. e 4. f 5. a 6. c 7. c 8. a 9. b 10. b 11. c 12. a 339
27.2 1. clade 2. Archaea, Bacteria, Eukarya (Eukaryotes) 3. horizontal gene transfer 4. ingroup 5. supergroup 6. synapomorphy 7. unweighted pair group method with arithmetic mean 27.3 1. molecular clock 2. orthologs 3. paralogs 4. nonneutral 5. neutral theory 6. Nearly neutral 7. Synteny 8. orangutans; 2; 3 ; inversion 27.4 1. BMP4 2. Gremlin 3. Hox 4. Cambrian 5. aniridia 6. eyeless 7. evo-devo Quiz 1. d 2. c 3. b 4. b 5. c 6. e 7. d 8. a 9. c 10. e 306